WO1997000978A1

WO1997000978A1 - Process for the manufacture of a high carbon cobalt-chromium-molybdenum alloy

Info

Publication number: WO1997000978A1
Application number: PCT/GB1996/001507
Authority: WO
Inventors: Graham Berry
Original assignee: Firth Rixson Superalloys Limited
Priority date: 1995-06-22
Filing date: 1996-06-21
Publication date: 1997-01-09
Also published as: GB9512719D0; GB2302551A; GB2302551B; AU6233396A

Abstract

A process for the manufacture of a high carbon cobalt-chromium-molybdenum alloy article which comprises: (i) subjecting a raw material charge of controlled composition to vacuum induction melting in a first furnace; (ii) casting the molten alloy from the first furnace into a first ingot; (iii) preparing the ingot into an electrode and subjecting the electrode to electroslag refining in a second furnace; (iv) casting the molten refined alloy progressively through the second furnace into a second ingot; (v) heat treating the second ingot to promote homogenisation thereof; (vi) forging the second ingot into a billet; and (vii) optionally, rolling the billet into a desired shape.

Description

PROCESS FOR THE MANUFACTURE OF A HIGH CARBON COBALT-CHROMIUM-MOLYBDENUM ALLOY

This invention relates to alloys, and more particularly to a novel method for the production of cobalt-chromium-molybdenum alloys, and alloys produced thereby. 5

Cobalt-chromium-molybdenum alloys, in the form of forged or machined components, have been used, for example, for the production of medical implants, for a number of years, and such products are covered by the

10 internationally recognised ASTM F799 and ASTM F1537 specifications. Established alloys in the market generally have a low carbon content, and forged and machined articles produced from such alloys have insufficient wear resistance and insufficient strength

15 for some applications. For some time attempts have been made to produce a high carbon (up to about .35 percent) cobalt-chromium-molybdenum alloy, but extreme difficulties have been experienced in making such alloys with the appropriate physical and chemical properties.

20 Typically, alloys which have so far been produced have extreme surface hardness and cause rapid wear of machining tools. Other such alloys are very brittle, and this is believed to be due to precipitation of carbon at the grain boundaries. Acceptable high carbon cobalt-

25 chromium-molybdenum alloys have been produced by powder metallurgical routes, but these are extremely expensive. It has been proposed, in East German patent number 148238, to manufacture a high carbon cobalt-chromium- molybdenum alloy containing nitrogen, by vacuum melting the charge, flushing the melting vessel with nitrogen, adding a chromium nitride to the melt and casting the molten metal into ingots in a nitrogen atmosphere. The ingots are then heat treated, forged into blanks, and rolled. This process is believed likely to give inadequate compositional control for many applications.

According to the present invention, high quality, high carbon, cobalt-chromium-molybdenum alloys are obtained by a melting and refining process which includes the step of electroslag remelting, followed by heat treatment and working under controlled temperature conditions.

In one aspect, the present invention provides a process for the manufacture of a high carbon cobalt- chromium-molybdenum alloy article which comprises:

1. subjecting a raw material charge of controlled composition to vacuum induction melting in a first furnace;

2. casting the molten alloy from the first furnace into a first ingot; 3. preparing the ingot into an electrode and subjecting the electrode to electroslag refining in a second furnace;

4. casting the molten refined alloy progressively through the second furnace into a second ingot;

5. heat treating the second ingot to promote homogenisation thereof;

6. forging the second ingot into a billet; and

7. optionally, rolling the billet into a desired shape.

Preferably steps 5, 6 and 7 are carried out at a temperature less than 1155°C and preferably less than 1145°C.

The raw material for the charge to the first furnace can comprise, for example, high purity virgin metals, and selected and processed alloy scrap. Individual charges are preferably prepared to controlled compositional specifications, and for example, suitable compositions can fall within the following preferred ranges: Composition Min ( % ) Max ( % ) Preferred

C 0.10 0.35 0.20

S 0.002 0.0005

B 0.010 0.006

Si 1.0 0.2

Mn 1.0 0.8

P 0.010 0.005

Co Balance Balance

Cr 26.0 30.0 28.5

Fe 0.75 0.50

Mo 5.0 7.0 6.0

Ni 1.0 0.80

Al 0.03 0.20 0.10

N 0.25 0.10

0 0.004 0.002

Some of the above compositions are new materials and are accordingly included within the invention.

In induction melting, an electric current is induced into a metal charge causing heating and subsequent melting. The induced current is produced, for example, by a primary current being passed through a water cooled copper coil, contained within a refractory lined furnace body. The metal charge which is contained within the refractory lining, in effect, becomes the core of the circuit. In the process of the present invention, vacuum induction melting is preferably conducted within a sealed chamber, from which all gases are exhausted by a vacuum pumping system. The charge may be loaded into the induction furnace, for example, through a vacuum-tight bulk charging port which is an integral part of the vacuum chamber shell. In the event that the furnace is air released, for example, at the start of a lining campaign, then the furnace can be charged directly. Preferably the furnace is provided with a vibratory chute system for the addition of small quantities of late additions and trimming alloys. When a satisfactory vacuum level has been achieved, power is applied and melting commences. Completion of the charging and melting process can then be progressive, via the bulk charging port.

At appropriate times during the vacuum induction melting process the bath of liquid metal in the furnace can be sampled, chemical analysis conducted, and the composition adjusted, by additions, for example, via the vibratory chute. A period of super heating or refining may be necessary at this stage depending on the specific alloy being produced, prior to bringing the liquid charge to the precise pouring temperature.

Preferably the vacuum induction melting step is carried out at a pressure of 15 microns vacuum or less, for example, from 1 to 5 microns, and, towards the end of the refine, a maximum pressure rise rate of 28 microns per minute. A typical melting cycle can last, for example, from 8 to 12 hours, preferably around 10 hours, and the molten bath temperature is from 1400°C to 1500°C, preferably around 1470°C.

After melting in the first furnace, the temperature of the bath is preferably raised to around 1500°C to 1600°C, for example around 1555°C, for pouring.

In casting the molten alloy from the first furnace, the furnace can, for example, be tilted and alloy dispensed either directly from the lip thereof, or via a "T" pot pouring tube, into a pre-cast refractory launder and tundish system, which may incorporate filters, baffles and weirs to maintain cleanliness. The alloy can then travel into tubular metal moulds, for example, of 23 or 33 cms diameter.

Preferably the final stages of the melting and casting process are carried out under an argon atmosphere which can, for example, comprise an argon back pressure of around 20mm but preferably at least 300mm. The use of an argon atmosphere helps to prevent the loss from the liquid charge of specifically added elements such as nitrogen, manganese, and magnesium. After removal from the casting chamber, the cast ingots can be stripped from the moulds and transferred to the electroslag refining stage.

In the electroslag refining process the alloy is further refined to a composition for subsequent application as a wrought product. The cast ingots can be prepared into electrodes, for example, by rough dressing of the surface and welding a "stub" on to one end. The prepared electrode can, for example, be suspended vertically in the second furnace, with the stub of the top forming one of the electrical contacts. The bottom of the electrode forms an electrical circuit through contact with a molten slag bath contained within the second furnace, which can, for example, comprise a water- cooled copper mould. The base plate of the mould can be supported on a vertical travel, and thereby complete the electrical circuit.

In the electroslag refining process, pre-mixed slag, comprising various constituents such as, for example, fluorspar and lime, is arc melted, transferred into the copper mould, the electrode presented, and power applied. Progressive melting then takes place with the bottom of the electrode melting, passing through the slag, and solidifying on the base plate. The electrode can be continually fed into the liquid slag, and the base plate continually withdrawn, forming a remelted ingot. The process can be automatically computer controlled with electrode feed, ingot withdrawal, cooling water flow and electrical current being balanced.

Preferably the electrode melt rate is from 3 to 4.5 kilogrammes per minute, the slag depth is from 80mm to 120mm, and the slag composition is around 70% calcium fluoride CaF₂ with the balance made up of calcium oxide CaO, magnesium oxide MgO, and alumina Al₂0₃.

The refined second ingot from the second furnace can, for example, be from around 30 to 45 cms in diameter. On the completion of remelting, the second ingot can be transferred to a cooling station from whence it is subjected to heat treatment and working.

Heat treatment of the second ingot is desirably carried out at a temperature and for a time sufficient to improve the homogeneity of the alloy. In a preferred process according to the invention, heat treatment is carried out at a temperature of from 1125°C to 1155°C, preferably around 1140°C for a minimum treatment time of at least 12 hours. Heat treatment may be carried out in any suitable furnace.

It is important that in the heat treatment and forging stages the temperature of the second ingot should not rise substantially above 1155°C, since if the temperature rises in an uncontrolled manner, this can lead to carbon precipitation at the grain boundaries. Such precipitation can lead to a very brittle end product.

After heat treatment, the second ingot is subjected to mechanical working, which preferably comprises forging and/or rolling.

The forging process can be carried out, for example, on an open-die forging press, or using an automated reciprocating process, or a combination of both. The maximum forging temperature is preferably 1140°C plus or minus 15°C, the minimum forging temperature is preferably 950°C plus or minus 15°C, and the minimum reduction achieved is preferably at least 50%. Careful control of the forging conditions is necessary to avoid cracking of the second ingot. The forging process can convert the second ingot into billet stock which can then be rolled if desired.

Rolling to finished size can be conducted, for example, on a rolling mill, to produce a final product which can, for example, be a round bar of diameter of from around 1 to 5cms, such as, for example, around 2 to 3.8 cms. The rolling conditions should be carefully controlled, and preferably the maximum rolling temperature is 1140°C plus or minus 15°C, the minimum rolling temperature is preferably 950°C, plus or minus 15°C, and the minimum reduction achieved is preferably at least 50%.

Preferred high carbon cobalt-chromium-molybdenum alloy articles in accordance with the invention can be produced having a microstructure of fine grain size of ASTM 5, preferably ASTM 8, the grain structure comprising finally divided, discrete, primary intragranular carbide particles.

Preferred minimum mechanical properties for an alloy produced in accordance with the invention can be as follows: Mechanical properties (room temperature) :

.2% P.S U.T.S R of A

(MPa) (MPa) El (%) (%) Hardness

(min) (min) (min) (min) HRc 720 1150 10 10 >35

High carbon cobalt-chromium-molybdenum alloys produced in accordance with the invention can be used, for example, for the manufacture of medical implants, either by machining directly from the rolled bar, or by using the rolled bar as forging stock to produce forged components of the required shape. A particular advantage of the process of the present invention is that, in preferred embodiments, the mechanical properties referred to above can be retained in the forged product, which is not the case for presently available F799 alloys. The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s) . This invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

1. A process for the manufacture of a high carbon cobalt-chromium-molybdenum alloy article which comprises:

(i) subjecting a raw material charge of controlled composition to vacuum induction melting in a first furnace;

(ii) casting the molten alloy from the first furnace into a first ingot;

(iii) preparing the ingot into an electrode and subjecting the electrode to electroslag refining in a second furnace;

(iv) casting the molten refined alloy progressively through the second furnace into a second ingot;

(v) heat treating the second ingot to promote homogenisation thereof;

(vi) forging the second ingot into a billet; and

(vii) rolling the billet into a desired shape.

2. A process according to Claim 1, in which steps (v), (vi) and (vii) are each carried out at a temperature of less than 1155°C.

3. A process according to Claim 1 or 2, in which steps (v), (vi) and (vii) are each carried out at a temperature of less than 1145°C.

4. A process according to any of the preceding claims, in which the raw material for the charge to the first furnace comprises a composition falling within the following ranges:

Composition Min (%) Max (%)

C 0.10 0.35

S 0.002

B 0.010

Si 1.0

Mn 1.0

P 0.010

Co Balance

Cr 26.0 30.0

Fe 0.75

Mo 5.0 7.0

Ni 1.0

Al 0.03 0.20

N 0.25

0 0.004 5. A process according to any of the preceding claims, in which the raw material for the charge to the first furnace has the following composition:

Composition

C 0.20

S 0.0005

B 0.006

Si 0.2

Mn 0.8

P 0.005

Co Balance

Cr 28.

5

Fe 0.50

Mo 6.0

Ni 0.80

Al 0.10

N 0.10

0 0.002

6. A process according to any of the preceding claims, in which the vacuum induction melting step is carried out at a maximum pressure of 15 microns vacuum.

7. A process according to any of the preceding claims, in which the vacuum induction melting step is carried out at a pressure of from 1 to 5 microns vacuum, and, towards the end of the refine, a maximum pressure rise rate of 28 microns/minute.

8. A process according to any of the preceding claims, in which the vacuum induction melting step is carried out for from 8 to 12 hours, and the molten bath temperature is from 1400°C to 1500°C.

9. A process according to any of the preceding claims, in which, after melting in the first furnace, the temperature of the bath is raised to 1500°C to 1600°C for pouring.

10. A process according to any of the preceding claims, in which at least the final stages of the vacuum induction melting step are carried out under an argon atmosphere.

11. A process according to any of the preceding claims, in which the first ingot is prepared into an electrode which is suspended vertically in the second furnace, with the bottom of the electrode contacting a molten slag bath contained within the second furnace.

12. A process according to Claim 11, in which pre-mixed slag, comprising fluorspar and lime, is melted and transferred into the second furnace.

13. A process according to any of the preceding claims, in which the electrode is melted, passes through the slag, and solidifies on a base plate of the second furnace.

14. A process according to Claim 13, in which the electrode is continually fed into the liquid slag, and the base plate continually withdrawn forming a remelted second ingot.

15. A process according to any of Claims 11 to 14, in which the electrode melt rate is from 3 to 4.5 kg/minute, and the slag depth is from 80mm to 120mm.

16. A process according to any of Claims 11 to 15, in which the slag composition comprises calcium

-.uoride CaF₂, with the balance made up substantially of calcium oxide CaO, magnesium oxide MgO, and alumina A1₂0₃.

17. A process according to any of the preceding claims, in which the second ingot is heat treated at a temperature of from 1125°C to 1155°C for a period of at least 12 hours.

18. A process according to any of the preceding claims, in which the second ingot is forged at a forging temperature of from 950°C +/- 15°C to 1140°C +/- 15°C and the reduction achieved is at least 50%.

19. A process according to any of the preceding claims, in which the forged billet is rolled at a rolling temperature of from 950°C +/- 15°C, to 1140°C +/- 15°C, and the reduction achieved is at least 50%.

20. A process according to any of the preceding claims in which the alloy article produced has the following minimum mechanical properties:

Mechanical properties (room temperature) :

.2% P.S U.T.S R of A

(MPa) (MPa) El (%) (%) Hardness

(min) (min) (min) (min) HRc

720 1150 10 10 >35

21. A process according to any of the preceding claims substantially as hereinbefore described.

22. A high carbon cobalt-chromium-molybdenum alloy article produced by a process according to any of the preceding claims.

23. A cobalt-chromium-molybdenum alloy having the following composition:

Composition Min (%) Max ( % )

C 0.10 0.35

S 0.002

B 0.010

Si 1.0

Mn 1.0

P 0.010

Co Balance

Cr 26.0 30.0

Fe 0.75

Mo 5.0 7.0

Ni 1.0

Al 0.03 0.20

N 0.25

0 0.004

24 A high carbon cobalt-chromium-molybdenum alloy substantially as hereinbefore described.